Slash notches are not as commonly used on commercial, store-bought sewing pattern so it is possible you have not yet come across one of them.
If you take a closer look at a slash marking notch on a sewing pattern, you will notice that it have an intersecting perpendicular line that forms a "T" shape. This additional line is crucial in the transfer process as it provides a stopping point for the slash. Any marking notch that is cut into the seam allowance, including triangle notches, should not extend past the midway point of the seam allowance. In this instance, the vertical "T" provides a safe guide to follow in the notch transfer process, thus preventing you from over-cutting into the seam allowance. The goal of and marking notch, is to be visible enough in the seam alignment process but not s=weaken or jeopardize the look and functionality of the finished seam.
Transferring T-shaped slash notches from paper pattern to fabric is very simple: Using the tip of your fabric scissors, carefully slash the marked line into the seam allowance, stopping at the vertical intersecting dash. It is important not to cut past this dash line as doing so may weaken the seam allowance layers and naturally, the final seam.
In seam alignment process, the two slashes should match perfectly. While slash notches are quite precise and simple to mark, they are not as visible as triangle shaped notches. You may have to pull the seam edges apart to find them along the fabric's cut edges, especially when working with textured fabrics. In this case, it helps to have more familiarity with the dressmaking process and use the slash notches as a way to cross check that your seams are indeed matched as they should be.
In some cases, you may have to carefully slash past the T line if your fabric is too bulky or highly fraying in order to leave a more visible mark behind. However, you should be very careful not to cut too close to the seamline as this may effect the finished seam's long term durability.